American bloggers have reacted angrily to proposals for a new law that could potentially make it illegal to criticise or make fun of somebody online.

Linda Sanchez, a Democratic congresswoman for California, is leading a bill intended to combat cyberbullying – but opponents say the law's limits are vague and threaten freedom of speech.

The bill, which is being submitted to Congress for the second time, proposes that any electronic communication intended to "coerce, intimidate, harass or cause substantial emotional distress" could be punished with a fine or a prison sentence of up to two years.

According the proposals, the new rules would cover email, blogs, instant messaging and texts.

Opponents are concerned that it could violate the US constitution's first amendment – which guarantees freedom of expression – and threatens valid online criticism, despite Sanchez's assertion that "ranting" would not become illegal.

But Eugene Volokh, a professor at the UCLA school of law and one of the most high-profile political bloggers in the US, has attacked the proposals as "overbroad" and "constitutionally vague".

Although serious cyberbullying is clearly an unwanted problem, Volokh says the definition of "severe" could easily lead to the censorship of campaigning blogs, political arguments or even consumer boycotts.

"This cannot possibly be constitutionally permissible, it cannot possibly be a good idea, it cannot possibly be what the drafters intended, and yet that is what they wrote," he said. "If it is passed through Congress, I see it being struck down in courts."

In Britain, authorities are equally concerned about the prospect of cyberbullying. Ed Balls, the secretary of state for children, schools and families, has previously called it "insidious" – and although there are potential legal safeguards in the Protection from Harassment Act and Suicide Act, no cases have yet reached court to form a precedent over online behaviour.

• This article was amended on Wednesday 6 May 2009. We misstated the party affiliation of Linda Sanchez. She is a Democratic congresswoman for California. This has been corrected.